Summary

Fish are an important part of aquatic ecosystems, with the structure of fish communities providing an indication of the overall health of river systems. In order to aid decision making, the Murrumbidgee Catchment Management Authority (CMA) funded a benchmarking survey of fish communities and threatened species within the Murrumbidgee catchment. Fish were sampled from 50 sites across the Murrumbidgee catchment.

The fish community is severely degraded with eight of the 21 native species which previously existed in the catchment either locally extinct or surviving at very low abundances. A ninth, trout cod, was extinct and has since been reintroduced. Only three of the seven threatened species known to have occurred in the Murrumbidgee were detected. It is highly likely that the remaining four species are locally extinct.

Introduced carp, eastern gambusia and redfin perch were three of the most widespread and abundant species in the catchment. Together these species made up 66% of individuals. Further, carp made up 87% of the total biomass (weight) of all fish sampled. The fact that they utilise 87% of the available fish resources within the catchment’s rivers identifies carp as the single largest feature of the current poor state of the catchment’s fish community and also the single largest factor preventing recovery to a more natural state.

Golden perch and Murray cod were the 2nd and 3rd most important species in terms of biomass and Australian smelt and carp-gudgeons were abundant. However, compared to the combined data for carp, redfin perch and gambusia, the values for these native species are very low. Silver perch and Macquarie perch were the two rarest species in the catchment. Silver perch are rare despite stocking just under 2 million fingerlings at 19 locations throughout the Murrumbidgee. No native species at all were sampled from 7 of the 28 (25%) sites, whereas alien fishes were only absent from 2 (7%).

Carp-gudgeons were the only native species whose population size has changed significantly over the last 10 years, with a consistent increase throughout the catchment. Of the remaining 13 native species assessed, eight (including Murray cod) had increased and five had declined. Redfin perch have increased in abundance consistently throughout the basin since 1994. The abundance of carp has varied, but the trends have been different in different parts of the catchment. Carp have declined in the lowland and slopes zones, but have increased in the upland and highland zones.

Without substantial intervention, the status of fish species and communities in the Murrumbidgee will not improve. Rehabilitation of instream and riparian vegetation, rehabilitation of wetlands, eliminating thermal pollution, improving environmental flow management, reinstating fish passage at a number of key barriers, contributing to the control of alien species, captive breeding and reintroduction programs for locally extinct populations and educating the community are avenues by which the Murrumbidgee CMA can rehabilitate fish communities in the Murrumbidgee catchment.